The Serial ATA interface specification defines a variety of requirements for serial communications between a host (e.g., a general purpose computer) and a device (e.g., a disk drive). When an equipment manufacture provides a product that complies with the Serial ATA interface specification, the equipment manufacturer knows that the product will be compatible with other complementary Serial ATA interface products (e.g., Serial ATA interface components provided by other equipment manufacturers).
To support Serial ATA communications, a power supply cable typically runs from a power supply (or power controller) of the host to the device to carry power supply and ground signals from the power supply to the device. Additionally, a separate Serial ATA communications cable typically runs from a controller or concentrator of the host (e.g., a host bus adapter, a RAID controller, etc.) to the device to carry information between the host and the device. Some equipment manufacturers leave the power supply cable and the Serial ATA communications cable separate by using a power supply connector (e.g., a legacy power supply connector) and a separate Serial ATA communications cable connector. Other equipment manufacturers combine the power supply cable and the Serial ATA communications cable into a single cable assembly by terminating both the power supply cable and the Serial ATA communications cable with a single integrated device connector which attaches to the device.
In general, the power supply cable includes four lines: a first power supply line for carrying a first voltage signal (e.g., 12 volts), a second power supply line for carrying a second voltage signal (e.g., 5 volts) and two ground lines. The ground lines provide return paths for the voltage signals. In some instances, more comprehensive power supply cables are used in order to provide additional voltage signals (e.g., 3.3 volts, 5 volts and 12 volts).
Similarly, the Serial ATA communications cable generally includes seven lines: two transmit lines (e.g., A+/A−) for carrying a differential mode transmit signal, two receive lines (e.g., B+/B−) for carrying a differential mode receive signal, and three ground lines. In both the host and the device, circuitry connected to one end of the Serial ATA communications cable converts information (e.g., bytes, words, etc.) into a serialized form for conveyance through the cable, and circuitry connected to the other end of the cable de-serializes the information back into parallel form for effective command, status and data transfer therebetween.
Further details of the Serial ATA interface specification are available in a document entitled “Serial ATA: High Speed Serialized AT Attachment”, Revision 1.0a, Jan. 7, 2003, the teachings of which are hereby incorporated by reference in their entirety. Additional details of the Serial ATA interface specification are provided in a document entitled “Serial ATA II: Extensions to Serial ATA 1.0”, Revision 1.0, Oct. 16, 2002, the teachings of which are hereby incorporated by reference in their entirety.
A conventional device presence detection mechanism for detecting whether a device is present is described in section 6.4.3 of the Serial ATA II document. This mechanism utilizes a precharge circuit and relies on the device busing together its power delivery contacts. As a result, there is either a small impedance between two points A and B on one power supply leg when no device is present, or no impedance across the two points A and B when the device is present.